In general, a wireless sensor network uses a small number of anchor nodes to estimate the positions of a plurality of mobile nodes, and is utilized for various applications, such as an application for locating a mobile node and an application for providing services suitable for the conditions of the mobile node.
Particularly, a position estimating algorithm uses the angle and distance between an anchor node and a mobile node, and the intensity of received signals. The distance is generally used, and is estimated from time of flight (TOF) that is estimated by ultrasonic waves or radio signals. In this case, synchronization between the anchor node and the mobile node is needed in order to obtain accurate distance information or TOF.
When a wide-area wireless network service such as a mobile phone service is provided, the supply of a network synchronization clock is very important to stabilize the operation of wide-area systems. The network synchronization clock is commonly supplied to various anchor nodes by wires.
On the other hand, in a temporary or local area network, such as a sensor network or an Ad hoc network, it is uneconomical to provide a separate network synchronization unit, or it is complicated or difficult to install the network synchronization unit.
Meanwhile, in the wireless sensor network according to the related art, in order to estimate the position of a mobile node, an active bat system and a cricket system use radio signals and ultrasonic signals. In this case, radio signals with a high propagation speed (about 3×108 m/s) are used for synchronization, and ultrasonic signals with a low propagation speed (about 340 m/s) are used to calculate TOF. According to the related art, a separate wire network synchronization signal is supplied from a control server that calculates position to anchor nodes through a wire cable with a predetermined length.
In the related art, there are a TWR (two-way ranging) method and a symmetric double sided (SDS)-TWR method that are capable of asynchronously calculating TOF without a synchronization clock, but a one-way ranging (OWR) method supported by network synchronization is strongly recommended with the merit of simple procedure. Further, the related art includes an active mode in which a mobile node transmits a ranging frame and a network estimates the position of the mobile node, and a passive mode in which anchor nodes transmit ranging frames and a mobile node estimates its position.
However, in the TWR and SDS-TWR methods according to the related art, estimate errors occur due to frequency offset between the nodes, three or four frame trans-missions are needed between one mobile node and each anchor node, and the number of frames to be transmitted increases. As a result, a long time is required for one position estimate, which causes various problems in providing services to a plurality of mobile nodes. For this reason, the OWR method that is effective after network synchronization has been proposed in the related art. However, the network synchronization and the OWR method are left on implementation dependent.